The present invention relates to an arrangement for cooling of recirculating exhaust gases of a combustion engine.
The technique called EGR (exhaust gas recirculation) is a known way of recirculating part of the exhaust gases in an exhaust line of a combustion engine. The recirculating exhaust gases are led through a return line and mixed with inlet air for the combustion engine before the mixture is led to the cylinders of the combustion engine. Adding exhaust gases to the air causes a lower combustion temperature resulting inter alia in a reduced content of nitrogen oxides NOx in the exhaust gases. This technique is used both for Otto engines and for diesel engines. The amount of exhaust gases which can be supplied to a combustion engine depends on the pressure and temperature of the exhaust gases. Supplying as large an amount of exhaust gases to the combustion engine as possible entails effective cooling of the exhaust gases before they are led to the combustion engine. A known practice is to cool the recirculating exhaust gases in one or more EGR coolers before they are led to the combustion engine.
During operation of combustion engines, e.g. diesel engines, the exhaust gases will be at a temperature ranging from 150° C. to 600° C. The temperature of the exhaust gases will vary with the load on the combustion engine. In cases where the combustion engine's cooling system is used for cooling the recirculating exhaust gases, the cooling system will thus be subject to high load peaks at times when the combustion engine is under great load. In heavy vehicles, the combustion engine's cooling system is commonly used also for other cooling requirements in the vehicle, e.g. cooling the oil of a hydraulic retarder. It is therefore desirable to reduce the load on the combustion engine's cooling system.
US 2008/0256949 refers to an arrangement for recirculation of exhaust gases whereby the recirculating exhaust gases undergo a first step of cooling in an air-cooled EGR cooler arranged in an engine space in the vehicle. To this end, a fan blows a cooling air flow through the EGR cooler. The recirculating exhaust gases are thereafter subjected to a second step of cooling by coolant in a low-temperature cooling system. In this case the air present in the engine space is used for subjecting the recirculating exhaust gases to a first step of cooling. As the recirculating exhaust gases may be at a very high temperature, the air at least locally in the engine space may be warmed to a very high temperature. The high air temperature may cause damage to nearby components in the engine space.
The exhaust line in a vehicle normally comprises one or more components for post-treatment of the exhaust gases before they are discharged to surrounding air. Such a component may be a catalyst adapted to removing nitrogen oxides NOx from the exhaust gases. However, most types of catalysts need to be at a lowest acceptable temperature for them to be able to effect optimum removal of nitrogen oxides NOx from the exhaust gases. An SCR (selective catalytic reduction) catalyst requires, for example, a temperature of about 200° C. Another component for post-treatment of the exhaust gases of diesel engines is a particle filter adapted to intercepting and burning soot particles in the exhaust gases. The particle filter likewise needs to be at a lowest acceptable temperature for it to function satisfactorily.
In supercharged combustion engines, the exhaust gases are led through a turbine. When the exhaust gases have given off energy in the turbine, they will be at a reduced pressure and a reduced temperature. In certain operating states of the combustion engine, the temperature of the exhaust gases after the turbine may be so low that the catalyst cannot maintain a lowest acceptable temperature and effect optimum cleaning of the exhaust gases.